Alas Surrey is a big county and covers parts of London, so rates can vary quite a bit area to area (especially if you are inside the M25).
One way of getting a feel for local market rates, is to search for nanny jobs like a nanny would. This will result in you seeing what salaries are being shown in job ads.
www.nannyjob.co.uk www.greatcare.co.uk are two sites to use.
As a nanny working in Surrey, I get around £19,000 gross pro-rata. However I started the job several years ago, and when I next change jobs, I would be looking at more like £23k+.
A local agency to me, recently told me that I should expect a salary of £22k to £26k.
There are extras that you will incur. Travel costs while on duty is one (40p per mile, if they use their own car), plus cost of activities, outings, etc. Nannies will typically have a weekly kitty for misc expenses - for activities and the such. If you budgeted £5 per day, then I expect it would be about right... though would depend on how much things cost in your area. Example, a music group near me costs £3.50 per session, swimming £3.80, Toddler group £2, PlayGym £3 (all based on adult + 2 under 5's). In some areas activity prices could be higher.
Budget for food which nanny will eat while on duty - lunch and tea (both to be eaten with the children) I would expect from the times you have given.
Employers NI will need to be taken into account. For simplistic purposes, lets take salary at £100 per day thus £300 per week. Employers NI is about £25.60 (07/08 tax year) see E-Gismos
Heating, Lighting costs may increase a bit, especially during winter months.
I would expect that with a nursery cost of £1400 per month... a nanny will probably work to be about the same. If it were £100 per day, then that is £1300 per month - to which you then need to add your extras, which would bump it up.
However, you get some benefits - a nanny may do the children's laundry, vacuum the house, empty the dish washer.